Reflecting on Mirror Mechanisms: Motor Resonance Effects during Action Observation Only Present with Low-Intensity Transcranial Magnetic Stimulation

Michela Loporto,David J Wright,Paul S Holmes,Craig J Mcallister

doi:10.1371/journal.pone.0064911

Michela Loporto, David J Wright + Show 2 more

Open Access

https://doi.org/10.1371/journal.pone.0064911

Copy DOI

Abstract

Transcranial magnetic stimulation (TMS) studies indicate that the observation of other people's actions influences the excitability of the observer's motor system. Motor evoked potential (MEP) amplitudes typically increase in muscles which would be active during the execution of the observed action. This ‘motor resonance’ effect is thought to result from activity in mirror neuron regions, which enhance the excitability of the primary motor cortex (M1) via cortico-cortical pathways. The importance of TMS intensity has not yet been recognised in this area of research. Low-intensity TMS predominately activates corticospinal neurons indirectly, whereas high-intensity TMS can directly activate corticospinal axons. This indicates that motor resonance effects should be more prominent when using low-intensity TMS. A related issue is that TMS is typically applied over a single optimal scalp position (OSP) to simultaneously elicit MEPs from several muscles. Whether this confounds results, due to differences in the manner that TMS activates spatially separate cortical representations, has not yet been explored. In the current study, MEP amplitudes, resulting from single-pulse TMS applied over M1, were recorded from the first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles during the observation of simple finger abductions. We tested if the TMS intensity (110% vs. 130% resting motor threshold) or stimulating position (FDI-OSP vs. ADM-OSP) influenced the magnitude of the motor resonance effects. Results showed that the MEP facilitation recorded in the FDI muscle during the observation of index-finger abductions was only detected using low-intensity TMS. In contrast, changes in the OSP had a negligible effect on the presence of motor resonance effects in either the FDI or ADM muscles. These findings support the hypothesis that MN activity enhances M1 excitability via cortico-cortical pathways and highlight a methodological framework by which the neural underpinnings of action observation can be further explored.

Highlights

Observing and understanding other people’s actions is crucial to our communication and social interactions
Experiment 2 tested whether action observation produced motor resonance effects, and if the choice of optimal scalp position (OSP) influenced the magnitude of these effects
The results presented here show that observing another person’s actions increases the excitability of the observer’s motor system and that this effect is selective to those muscles that would be involved in the execution of the observed actions

Summary

Introduction

Observing and understanding other people’s actions is crucial to our communication and social interactions. Termed mirror neuron system (MNS), which includes the premotor cortex, parietal areas and the superior temporal sulcus [5] has been proposed as the system responsible for many aspects of social cognition [6]. This network is thought to allow visual information from observed actions to be mapped onto the observer’s motor system, causing the observer’s brain to simulate the observed action [7]. Neuroimaging studies have shown similar neural representations between observation and execution (for a review see [8,9]), reinforcing the proposal of a human MNS

Methods

Results

Conclusion